Simplified safety: fail-safe functionality for hazardous environments

Installing fail-safe machine control systems in areas that have explosive atmospheres has been a problem because the products normally used in most machine-control equipment may cause sparks when disconnected. Any such sparks in an explosive atmosphere are dangerous to life, limb, and equipment.

The ultimate solution is to redesign the equipment to limit voltages and currents to levels where generation of sparks is impossible. That is, of course, good news for everyone, from the fire department in the surrounding community to the maintenance worker at the potential point of ignition. No disaster is a good disaster!

It sounds easy, but putting this concept into practice requires completely redesigning the control electronics from the semiconductors up. This makes the equipment intrinsically safe, meaning that explosion proofing is not an add-on. It’s not something like a shield that could be defeated or fail. It’s part of the equipment’s basic design.

In intrinsically safe equipment, all voltages and currents are limited to values that ensure that sparks cannot be generated. In addition, all electronics contain circuit elements, such as capacitors or MOVs in parallel with switch contacts to absorb kickback energy when interrupting currents in inductive circuits, and to bleed static charges that may build up anywhere within the equipment.

Limiting voltages and currents, of course, limits the energy available to form kickbacks to levels where the atmosphere itself is able to snuff sparks before they can appear. This is what is meant by “intrinsically safe.”

Intrinsically safe: No failure mode

The advantage of intrinsically safe control components is that there is no failure mode. You can’t get a spark because there is nothing there capable of making a spark. The probability of an explosion collapses to zero for all conditions.

Previously, engineers designing installations where explosive atmospheres could exist have had to set up a safe barrier to protect the dangerous area. While the term “safe barrier” calls to mind images of heavy concrete walls, it’s not like that at all.

Of course, there are walls around areas where explosive atmospheres gather. That’s what makes combustible gases, vapors or dust build up to explosive levels in the first place. Those walls, however, are the problem, not the solution.

The safe barrier solution is, instead, a set of electronic system components that reduce signals to safe, spark-free levels before they enter the hazardous area. Figure [safe barrier] illustrates the concept. Control-system components outside of the barrier all operate at standard voltage, current and power levels.

Incorporating fail-safe components into intrinsically safe control systems eliminates the need for safe barriers. That drastically reduces wiring, cabling, and engineering costs. The reason combining intrinsically safe technology with fail-safe technology is so important is that fail-safe devices, especially, can be located as close to the processes they control as possible. That means putting fail-safe devices within the hazardous atmosphere. That is impossible without including intrinsically safe technology as well.

Explosive atmosphere applications

Hazardous-atmosphere issues arise in many industries, such as chemical, petrochemical, oil and gas, even farming, where organic dust is famous for exploding with no warning. Wherever you have the manufacture, processing, transport or storage of combustible materials, you typically have creation or release of gases, vapors or mist into the environment. There are other processes that create combustible dust, such as grinding and milling, where you’re creating granular solids or dust.

Today, there are intrinsically safe fail-safe devices manufactured for use in explosive-atmosphere environments. These are the first generation of modular solutions for I/O in machine safety loops in hazardous areas, such as the modular distributed SIMATIC ET200 iSP, which is an I/O solution designed for installation in hazardous areas. The ET 200iSPO is programmed in a similar way to the standard SIMATIC PLC and distributed I/O modules with STEP 7 software over PROFIBUS. All these modules can be replaced during operation even under explosive conditions. If technicians need to replace something, they can just go pull out a module, and put a similar module in there, and continue with the operation thus reducing downtimes and further increasing productivity.

Overall, the benefits of combining intrinsically safe technology with machine safety include: no safe barriers required, reduced wiring, one cable carries both standard and safe communications, in-built HART communications, hot swapping, reconfiguration in run mode, integrated diagnostics. No barriers, less wiring, less cabling, less engineering translate directly into less installation and operation costs. Now, hazardous area application designs can be further simplified and made more efficient with the use of a fail-safe intrinsically safe control system solution.

John D’Silva is marketing manager for Safety Integrated solutions at Siemens Industry. Contact him at john.dsilva@siemens.com

For more information on safety automation related products, visit the Siemens Industry website at www.siemens.com.

www.usa.siemens.com/safety

This article was submitted for the Siemens Simplified Safety custom newsletter. See other articles in the Siemens Simplified Safety newsletter.

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